DUAL VOLTAGE VOLTAGE REGULATOR WITH FOLDBACK CURRENT LIMITING

Abstract: A voltage regulator is switchable between a lower regulated DC output voltage and a higher regulated DC output voltage. Foldback current limiting is actuated in response to the current drawn by the load when a current limiting threshold is exceeded. The current limiting threshold is determined by the voltage relationship at respective taps of a pair of voltage dividers, with said relationship being effected by the voltage appearing across a current sensing resistor coupled in series with the load. The current limiting threshold is adjusted to be approximately the same for both the lower and the higher regulated DC output voltages. This adjustment is accomplished with a non-linear voltage dependent device, which is coupled to one of the voltage dividers, and is actuated in the higher output voltage mode.

Description: DUAL VOLTAGE VOLTAGE REGULATOR WITH FOLDB ACK CURRENT LIMITING CROSS REFERENCE TO RELATED APPLICATIONS This application is related to a patent application docket No. 87,648, entitled "Voltage Regulator", concurrently filed herewith. BACKGROUND The present invention concerns voltage regulators, and more particularly, to a dual voltage voltage regulator with foldback current limiting wherein the threshold for initiating current limiting is maintained at approximately the same output current for each of the output voltages. Voltage regulators, which use a controllable series impedance device for maintaining a regulated output voltage coupled to a load, are susceptible to damage if a short circuit or other fault is applied to the output terminals of the regulator. Such damage often is caused by excessive thermal dissipation of the series impedance device or by greatly exceeding the current rating of the series device. For this reason, it is common to provide overload protection to prevent such damage to the regulator. One type of overload protection is current limiting in what is known as a "foldback" voltage regulator, such as is disclosed in U.S. Patent No. 3,445,751 of Easter. Such a regulator provides output voltage regulation for a changing load until an overload current threshold is reached. For load currents above this threshold, the available output current decreases as the load increases, with a corresponding decrease in the output voltage. The short-circuit current can be adjusted to be but a small fraction of the full load current, thus minimizing the dissipation in the series pass transistor. The voltage regulator of the present invention is such a "foldback" voltage regulator. Some applications require a voltage regulator which is capable of providing multiple output voltages. Accordingly, it is desirable to provide a multiple voltage voltage regulator having current limiting overload protection for both output voltage settings. SUMMARY OF THE INVENTION Briefly, the present invention concerns a voltage regulator which is switchable between a lower regulated DC output voltage and a higher regulated DC output voltage. Foldback current limiting is actuated in response to the current drawn by the load when a current limiting threshold is exceeded. The current limiting threshold is determined by the voltage relationship at respective taps of a pair of voltage dividers, with said relationship being effected by the voltage appearing across a current sensing resistor coupled in series with the load. The current limiting threshold is adjusted to be approximately the same for both the lower and the higher regulated DC output voltages. This adjustment is accomplished with a switching device, which is coupled to one of the voltage dividers, and is actuated in the higher output voltage mode. BRIEF DESCRIPTION OF THE DRAWING Reference can be had to the drawing which shows a schematic of the present regulator according to aspects of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figure 1, there is shown a voltage regulator 10 according to aspects of the present invention. Voltage regulator 10 can be switchable between a higher regulated DC output voltage mode and a lower regulated DC output voltage mode. An unregulated direct current power supply source (not shown) is connected between terminal 12 and a reference potential point 11 (e.g., ground). The emitter electrode 14 of series pass PNP transistor QI is coupled to terminal 12. The collector electrode 16 of transistor QI is coupled to an output terminal 18 through resistor 20. A load (LNB) is coupled between output terminal 18 and reference point 11 (not shown). The base electrode of transistor QI is coupled to a collector electrode of NPN amplification transistor Q2 and to input terminal 12 through a resistor 22. The emitter electrode of transistor Q2 is coupled to output terminal 18 through a resistor 24 and to reference point 11 by resistor 30. The base electrode of transistor Q2 is coupled to receive a control signal, which will be discussed more fully below. Supply current flows from the DC supply source coupled to terminal 12 through the emitter-collector path of transistor QI and resistor 20 to output terminal 18 and the load. The amount of this current is controlled by the control signal coupled to the base electrode of transistor Q2 via line 26, with the voltage drop across transistor QI being adjusted to maintain a regulated output voltage at terminal 18. A resistor 32, coupled between the emitter and collector electrodes of QI, continues to provide some current to the load even if transistor QI is completely cut-off. Resistor 22, coupled between the emitter electrode and the base electrode of transistor QI, reduces the effects of collector to base leakage currents in transistor QI. The complementary arrangement of transistors QI, Q2 provides both voltage and current gain since the collector electrode of transistor Q2 is coupled to the base electrode of transistor QI and the output of the series pass arrangement is taken from the collector electrode 16 of transistor QI. Thus, transistors QI, Q2 are arranged as amplifiers within a feedback loop with the loop gain determined by a feedback network comprised of resistor 24 coupled from output terminal 18 to the emitter electrode of transistor Q2, and resistor 30 coupled to ground. To have the regulator operate with a lower difference voltage between the input voltage Vin and the output voltage Vo, and reduce power dissipation in transistor QI, it is desirable that transistor QI be driven into saturation at the highest output voltages in the high voltage mode. Voltage divider resistors 24, 30 improve the efficiency of the series pass circuit to achieve these attributes. Voltage V26, at line 26, is mathematically expressed as follows: V26=Vbe of Q2+Vo(resistor 30/(resistor 30+resistor 24)). If the Vbe of Q2 is 0.7 volts and the value of resistor 24 equals the value of resistor 30, then: V26 = 0.7 volts + Vo/2. Since this arrangement lowers the voltage at the emitter of transistor Q2 to substantially below the voltage Vo, it makes it easier to drive Q2 harder since the voltage V26 can be a lower voltage, thus allowing transistor QI to be more easily driven into saturation while still maintaining transistor Q2 in an active non- saturating state. Thus, with divider resistors 24, 30, the series pass transistor QI can be driven so that Vo = Vin - 0.2 volts (the typical saturation voltage for transistor QI) instead of at least 1.4 voltage, as discussed above. Thus, the regulator can operate with a lower difference between the input voltage Vin and the output voltage Vo, and with a resulting reduction in the power dissipation in transistor QI when it is fully driven. The lower difference between input and output voltages is of particular importance in the higher output voltage mode because the maximum value of voltage Vin is limited. Additionally, since the control voltage applied to lead 26 is now considerably lower than B+, operational amplifier 46, which provides control signal V26, as will be discussed more fully below, is not required to operate at output voltages near the value of B+ in order to drive transistor Q2 to saturate transistor QI . A resistor 28 is coupled between the emitter electrode 14 of transistor QI and the emitter electrode of transistor Q2, to prevent the emitter electrode of Q2 from falling so low when the output is short circuited, that operational amplifier 46 cannot reverse bias the base-emitter junction of transistor Q2 to cut-off transistor QI. The ability to cause transistor QI to be cut-off is important for current limiting, which will be discussed more fully below. A reference voltage is provided by resistor 34 and zener diode 36 connected in series between input terminal 12 and ground, and the reference voltage is filtered by a capacitor 38. The reference voltage is coupled to a non-inverting (ni) input terminal 46ni of an operational amplifier 46 where it is compared to a divided down version of Vo, which is coupled to an inverting (i) input terminal 46i. The divided down version of Vo is derived from a tap at the junction of series voltage divider resistors 42 and 44 coupled between output terminal 18 and ground 11. The output signal of amplifier 46 provides the control signal V26 at line 26 through isolation resistor 50. This arrangement provides negative feedback which reduces or increases the drive to transistor QI if there is a respective increase or decrease in the regulated output voltage Vo. Capacitor 49, coupled between the output of amplifier 46 and terminal 46i, suppresses oscillation. Switching between lower and higher output voltage modes is made possible by transistor Q3, which can be driven into saturation by a control signal coupled to its base electrode from a control unit, (not shown), such as a microprocessor, through resistor divider 51, 52. The collector electrode of transistor Q3 is coupled to terminal 46i by resistor 54, and when transistor Q3 is driven into saturation, resistor 54 is coupled in parallel with divider resistor 44, thus modifying the voltage divider ratio of resistors 42, 44. The resulting change in V26, provided by comparator amplifier 46, causes the output voltage at terminal 18 to be switched to the higher voltage. Turning now to the foldback current limiting aspect of the present regulator, a voltage divider 58, comprising series resistors 60, 62 and 64, is coupled between collector 16 of transistor QI and ground, with a tap at the junction of resistors 62 and 64 being coupled to an inverting input terminal 66i of operational amplifier 66. A voltage divider 68, comprising series resistors 70 and 72, is coupled between output terminal 18 and ground, with a tap at the junction of the resistors 70, 72 being coupled to a non-inverting (ni) input terminal 66ni of amplifier 66. Output terminal 74 of amplifier 66 is coupled to the cathode of a diode 76, with the anode of diode 76 being coupled to control lead 26. Diode 76 prevents operational amplifier 66 from effecting V26 during normal operation, as will be discussed more fully below. Capacitor 79, coupled between output terminal 74 and terminal 66i, suppresses oscillation. Capacitor 80, coupled across resistor 72, prevents any AC signal received from the LNB load from effecting amplifier 66. The component values of the resistors in dividers 58, 68, are as follows: resistor 60 = IK ohms resistor 62 = 3K ohms resistor 64 = 12K ohms resistor 70 = 2.8K ohms resistor 72 = 12K ohms Resistor 20, (3.3 ohms), develops a voltage thereacross proportional to the output current. Thus, the voltages across dividers 58 and 68 are slightly different, and the voltages at the taps of the two dividers are arranged to be slightly different. When current drawn through resistor 20 is less than the threshold foldback current, the action of voltage dividers 58 and 68 is such that the voltage at terminal 66ni is more positive than the voltage at terminal 66i, and the output voltage at terminal 74 is at or near the B+ voltage. This back biases diode 76 and prevents the output of amplifier 66 from interfering with the drive at line 26 under normal operation. Thus, unless the circuit is in the current limiting mode, normal control of line 26 is provided by amplifier 46. However, if the current drawn through resistor 20 exceeds the foldback threshold current, the voltage drop across resistor 20 causes the voltage at the terminal 66ni to be slightly lower than the voltage at terminal 66i. This forces the output voltage at terminal 74 to go low due to the large gain of operational amplifier 66. This causes diode 76 to be forward biased and cause the operation of amplifier 46 to be overriden so that the control voltage on line 26 is reduced to nearly zero volts. As a result, the output current at terminal 18 is reduced to nearly zero and output voltage Vo is reduced to nearly zero volts. In this manner, when the output is short circuited or a fault occurs in the load, the output current is "folded back" from the nominal output current which is provided to the load during normal operation. For example, the output current may be folded back from a normal value of 350 milliamperes to about 10 milliamperes. Thus, transistor QI is protected from being subjected to excessive thermal dissipation or overcurrent condition due to a load fault. When the load fault is removed, voltage regulator 10 recovers and returns to normal operation. Voltage regulator 10 is a dual voltage voltage regulator. When the output voltage Vo is changed to the higher voltage, the foldback threshold current at which current limiting is initiated, would also be changed. The change in the foldback threshold current occurs because the voltage drop across the current sensing resistor 20 would remain the same for any particular current, but the differential voltage coupled to input terminals 66ni and 66i due to the increase in voltage across voltage dividers 58, 68. This is not desirable since the protection afforded transistor QI and the load would be reduced. In the present embodiment, to maintain the same current limiting threshold in the higher voltage mode, the voltage division of divider 58 is altered by diode 78 coupled across resistor 60. The voltage drop across resistor 60 is chosen to be less than the threshold of forward conduction of diode 78 in the lower output voltage mode. However, when regulator 10 is switched into the higher voltage mode, the higher voltage drop across resistor 60 is sufficient to cause diode 78 to conduct in its forward direction, thus changing the voltage division of divider 58 and the relationship of the difference voltage applied to terminals 66i and 66ni. This change of voltage divider 58 maintains substantially the same foldback threshold current in the higher voltage output mode as in the lower voltage output mode. For example, without the change in voltage divider 58, the current limiting threshold at the lower regulated output voltage, in the exemplary embodiment, would be about 350 ma, and the current limiting threshold at the higher regulated output voltage would be about 600 ma. With the change in voltage divider 58, the current limiting threshold is about 350 ma for each of the dual output voltages. In the present embodiment, diode 78 is a 1N914 diode having a reasonably sharp "knee". If it is desired to reduce the sharpness of the conduction knee, a resistor (not shown) can be connected immediately in series with diode 78. Alternately, diode 78 can be replaced by a plurality of series connected diodes. Other voltage sensitive devices can also be used, such as germanium diodes, LED's, voltage dependent resistors, or zener diodes. In the case of an LED, the diode itself may be a visual indicator as to the operating mode of the regulator. Additionally, a relay or a switching transistor can be used in place of diode 78. In such a case, the presence or absence of a microprocessor signal, such as available at terminal 53, can be used to initiate the switching of the divider resistors when that same microprocessor signal initiates the change in output voltage. Still further, the voltage sensitive device can be connected elsewhere in one of the voltage dividers. It should be noted that in the exemplary embodiment, operational amplifiers 46 and 66 are LM348 operational amplifiers made by National Semiconductor of USA. These operational amplifiers have PNP input circuits which permit the amplifiers to still be operational when the voltages at the input terminals are very low. However, it has been found that operational amplifiers having NPN input circuits, typically are not operational when the voltages at the input terminals are lower than about one volt. It has been found that if such NPN input circuit operational amplifiers are used, the amplifier 66 may latch in the foldback current limiting mode, i.e., output terminal 74 is latched to zero output volts, and will not recover to a normal operating mode when the fault is removed from output terminal 18. However, there may be situations where this latching in a "fail-safe" mode may be desirable. The present voltage regulator is useful in a direct broadcast satellite receiver system which includes an outdoor microwave antenna which can be aimed at a satellite to receive a signal from the satellite. The signal received from the satellite is amplified by a "low noise block converter" (LNB) mounted in very close proximity to or on the antenna. The output signal from the LNB is carried to an indoor receiver by a coaxial cable. In order to supply power from the indoor receiver to the LNB, as well as to control the polarization of the LNB, a DC voltage is multiplexed onto the center conductor of the coaxial cable. The circuits in the LNB are designed so that they will function with either a lower power supply voltage or a higher power supply voltage, with the dual supply voltages being used to control polarization settings of the LNB, e.g., the lower voltage selecting right hand circular polarization (RHCP) and the higher voltage selecting left hand circular polarization (LHCP). The current drain of the LNB is fairly constant with either of the regulated power supply voltages. The multiple output voltage current limiting arrangement described above is well suited for a power supply which provides multiple voltages to an LNB because of safety features provided by the power supply. However, the invention is not limited to such an application.

Claims: CLAIMS 1. A voltage regulator providing a plurality of regulated output voltages, and current limiting for each of said plurality of voltage settings, comprising: an input terminal for receiving a unregulated DC input voltage; an output terminal for providing a DC output voltage; means responsive to a control signal and coupled between the input terminal and the output terminal for regulating said DC output voltage; means for changing the control signal in response to the magnitude of the regulated DC voltage, the magnitude of the control signal also being switchable for providing a first and a second regulated DC voltages at the output terminal; a first sensing means including a first voltage divider for providing a first sensed voltage corresponding to the value of the regulated DC voltage, a second sensing means including a second voltage divider for providing a second sensed voltage corresponding to the value of the current drawn by the load; means responsive to the first and second sensed voltages for limiting the current supplied to the load when the magnitude of said current drawn by the load exceeds a threshold value; and means coupled to one of the first and second voltage dividers for changing the one of the first and second sensing voltages when the regulated DC voltage is switched between the first and second regulated DC voltages. 2. The regulator of claim 1 wherein the means coupled to one of the first and second voltage dividers is a diode poled to conduct when the regulated DC voltage is switched to the higher of the first and second regulated DC voltages. 3. The regulator of claim 1 wherein the means responsive to the sensed voltages generates a signal for modifying the control signal. 4. A voltage regulator providing a plurality of regulated output voltages and current limiting for each of said plurality of voltage settings, comprising: an input terminal for receiving a unregulated DC input voltage; an output terminal for providing a DC output voltage; means responsive to a control signal and coupled between the input terminal and the output terminal for regulating said DC output voltage; means for changing the control signal in response to the magnitude of the regulated DC voltage, the magnitude of the control signal also being switchable for providing a first and a second regulated DC voltages at the output terminal; a first sensing means including a first voltage divider for providing a first sensed voltage corresponding to the value of the regulated DC voltage, a second sensing means including a second voltage divider for providing a second sensed voltage corresponding to the value of the current drawn by the load; means responsive to the first and second sensed voltages for limiting the current supplied to the load when the magnitude of said current drawn by the load exceeds a threshold value; and means coupled to one of the first and second voltage dividers for changing the one of the first and second sensing voltages when the regulated DC voltage is switched between the first and second regulated DC voltages, said means being a voltage dependent device made conductive when the output DC regulated voltage is switched to the higher of the first and second output voltages. 5. The regulator of claim 4 wherein the voltage dependent device is a diode poled to conduct when the regulated DC voltage is switched to the higher of the first and second regulated DC voltages. 6. The regulator of claim 4 wherein the means responsive to the sensed voltages generates a signal for modifying the control signal. 7. A voltage regulator providing a plurality of regulated output voltages and current limiting for each of said plurality of voltage settings, comprising: an input terminal for receiving a unregulated DC input voltage; an output terminal for providing a DC output voltage; means responsive to a control signal and coupled between the input terminal and the output terminal for regulating said DC output voltage; means for changing the control signal in response to the magnitude of the regulated DC voltage, the magnitude of the control signal also being switchable for providing a first and a second regulated DC voltages at the output terminal; a first sensing means for providing a first sensed voltage corresponding to the value of the regulated DC voltage, a second sensing means for providing a second sensed voltage corresponding to the value of the current drawn by the load; means responsive to the first and second sensed voltages for limiting the current supplied to the load when the magnitude of said current drawn by the load exceeds a threshold value; and means coupled to one of the first and second sensing means for changing one of the sensing the regulated DC voltage and the current provided by the regulated DC voltage to the load when the regulated DC voltage is switched between the first and second regulated DC voltages. 8. The regulator of claim 7 wherein the first and second sensing means are respective first and second voltage dividers, and the means coupled to one of the first and second voltage dividers is a diode poled to conduct when the regulated DC voltage is switched to the higher of the first and second regulated DC voltages. 9. The regulator of claim 7 wherein the means responsive to the sensed voltages generates a signal for modifying the control signal. 10. The voltage regulator of claim 7 wherein: the regulating means comprising a first transistor of a first type and having a first emitter electrode coupled to the input terminal, a first base electrode, and a first collector electrode coupled to the output terminal, and a second transistor of a type complementary to the type of the first transistor and having a second base electrode coupled for receiving the control signal, a second emitter electrode coupled to the output terminal, and a second collector electrode coupled to the first base electrode. 11. The voltage regulator of claim 10 further comprising means coupled to the second emitter electrode for maintaining the second emitter electrode at a voltage which is less than the regulated DC voltage at the output terminal. 12. A voltage regulator for supplying a regulated voltage to a low noise block converter (LNB) of a satellite receiver system comprising: means for providing a first and second regulated output voltages to the LNB, said second regulated voltage being different from the first regulated voltage, and means for providing first and second magnitudes of current limiting for the respective first and second regulated output voltages, the first and second magnitudes of current limiting being substantially the same. 13. A voltage regulator providing a plurality of regulated output voltages and current limiting for each of said plurality of voltage settings, comprising: an input terminal for receiving a unregulated DC input voltage; an output terminal for providing a DC output voltage; means responsive to a control signal and coupled between the input terminal and the output terminal for regulating said DC output voltage; means for changing the control signal in response to the magnitude of the regulated DC voltage, the magnitude of the control signal also being switchable for providing a first and a second regulated DC voltages at the output terminal; sensing means including first and second sensing means for respectively sensing the regulated DC voltage and the current provided by the regulated DC voltage to the load; said sensing means in response to the sensed current, limiting the current supplied to the load when the magnitude of said current exceeds a threshold value; means coupled to one of the first and second sensing means for changing one of the sensing means when the regulated DC voltage is switched between the first and second regulated DC voltages, the regulating means comprising a first transistor of a first type and having a first emitter electrode coupled to the input terminal, a first base electrode, and a first collector electrode coupled to the output terminal, and a second transistor of a type complementary to the type of the first transistor and having a second base electrode coupled for receiving the control signal, a second emitter electrode coupled to the output terminal, and a second collector electrode coupled to the first base electrode, and means coupled to the second emitter electrode for maintaining the second emitter electrode at a voltage which is less than the regulated DC voltage at the output terminal. 14. The voltage regulator of claim 13 wherein the sensing means generates a signal for modifying the control signal.

Inventors: MUTERSPAUGH MAX WARD, MUTERSPAUGH, MAX, WARD
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS, INC.
Application year: 1994
Main classification: G05F1
IPC codes: G05F1, H03F1